This invention is related to novel methods for manufacturing components and structures, such as metal components and structures, and more particularly for parts having apertures therein, or cutouts therein, and which parts are subject to repeated or prolonged stress, in order to improve structural integrity by providing improved resistance to metal fatigue. Such methods, and the apparatus made thereby, have widespread applications in components and structures for transportation systems, and for medical devices (particularly metal parts having apertures therein). More specifically, the invention is applicable to fabrication of structures and components having apertures designed for accommodating fasteners such as rivets and bolts, as well as for those for routing tubing, cable or wires (including, for example for fuel flow), or those apertures simply provided for weight reduction purposes, so that finished parts and the apparatus in which the components or structures are installed, have improved resistance to metal fatigue and consequently better structural integrity.
Metal fatigue is a problem common to just about everything that experiences cyclic stresses. Such problems are especially important in transportation equipment, such as aircraft, ships, trains, cars, and the like. Metal fatigue can be defined as the progressive damage, usually evidenced in the form of cracks, that occurs to structures as a result of cyclic loading. This failure mode is not to be confused with a failure due to overload. The lower surface of an aircraft wing is a classical example of the type of loading that produces fatigue. The wing is subjected to various cyclic stresses resulting from gust, maneuver, taxi and take-off loads, which over the lifetime of a particular part eventually produces fatigue damage. Similarly, the pressurized envelope of an aircraft, including the fuselage skin and rear pressure bulkhead, are subject to a stress cycle on each flight where the aircraft interior is pressurized.
One problem inherent in fatigue damage is that it can be hidden since it generally occurs under loads that do not result in yielding of the structure. Fatigue damage is most often observed as the initiation and growth of small cracks from areas of highly concentrated stress. Undetected, a crack can grow until it reaches a critical size. At that point, the individual structural member can suddenly fail. Catastrophic failure of an entire structure can also occur when other members of the adjacent portions of the overall structure can not carry the additional load that is not being carried by the failed structural member.
Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. Cyclic loads for railroad tracks are caused by repeated loading from the wheels running over an unsupported span of track. In fact, some of the earliest examples of fatigue failures were in the railroad industry and in the bridge building industry. Also, sudden pressure vessel failures can be caused by fatigue damage that has resulted from repeated pressurization cycles. Importantly, government studies report that fatigue damage is a significant economic factor in the U.S. economy.
Fatigue can be defined as the progressive damage, generally in the form of cracks, that occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), fillets, radii and other changes in structural cross-section, as at such points, stress is concentrated. Additionally, such points often are found to contain small defects from which cracks initiate. Moreover, the simple fact that the discontinuity in a structural member such as a fuselage or wing skin from a hole or cutout forces the load to be carried around the periphery of such hole or cutout. Because of this phenomenon, it is typically found that stress levels in the material adjacent to fastener holes or cutouts experience stress levels at much greater than the nominal stress which would be experienced at such location, absent the hole or cutout.
It is generally recognized in the art that the fatigue life in a structure at the location of a through aperture or cutout can be significantly improved by imparting beneficial residual stresses around such aperture or cutout. Various methods have been heretofore employed to impart beneficial residual stress at such holes or cutouts. Previously known or used methods include roller burnishing, ballizing, split sleeve cold expansion, split mandrel cold working, shot peening, and pad coining. Generally, the compressive stresses imparted by the just mentioned processes improve fatigue life by reducing the maximum stresses of the applied cyclic loads at the edge of the hole. Collectively, these processes have been generically referred to as cold working. Basically, the presently known methods of cold working holes and other cutouts using tapered mandrel methods, coining, punching, and such are not adaptable to automated fastening systems and other automated environments because of their complexity and bulkiness of equipment. Also, presently known methods used by others do not treat the entire periphery of non-circular cutouts leading to potential fatigue life degradation. Finally, prior art countersink cold working methods require re-machining of the formed countersink, in order to achieve the desired fastener flushness.
Shortcomings of currently known methods for treating structures to provide enhanced fatigue life will be used as a basis for comparison with my novel, improved stress wave fabrication method. Heretofore known processes are not entirely satisfactory because:
they generally require that a starting hole be created in a workpiece, prior to initiating a stress fatigue life improving process;
they often require mandrels, split or solid, and disposable split sleeves, which demand precision dimensions, which make them costly;
mandrels and sleeves are an inventory and handling item that increases actual manufacturing costs when they are employed;
xe2x80x9cmandrelxe2x80x9d methods require a different mandrel for roughly each 0.003 to 0.005 inch change in hole diameter, since each sleeve is matched to a particular mandrel diameter, and consequently, the mandrel system does not have the flexibility to do a wide range of hole existing hole diameters;
each hole diameter processed with xe2x80x9cmandrelxe2x80x9d methods requires two sets of reamers to finish the hole, one for the starting dimension and another for the final dimension;
mandrel methods rely on tooling and hole dimensions to control the amount of residual stress in the part, and therefore the applied expansion can be varied only with a change of tooling;
mandrel methods require some sort of lubricant; such lubricants (and especially liquid lubricants), often require solvent clean up;
splits in a sleeve or splits in a mandrel can cause troublesome shear tears in certain 7000 series aluminum alloys;
the pulling action against mandrels, coupled with the aperture expansion achieved in the process, produces large surface marring and upsets around the periphery of the aperture;
split sleeve methods are not easily adapted to the requirements of automation, since the cycle time is rather long when compared with the currently employed automated riveting equipment;
mandrel methods are generally too expensive to be applied to many critical structures such as to aircraft fuselage joints, and to large non-circular cutouts;
mandrel methods have limited quality control/quality assurance process control, as usually inspections are limited to physical measurements by a trained operator.
My novel stress wave manufacturing process can be advantageously applied to apertures for fasteners, to large holes in structures, to countersunk holes, to non-round cutouts from a workpiece, and to other structural configurations. Treating a workpiece structure for fatigue life improvement, prior to fabricating the aperture itself, has significant technical and manufacturing cost advantages. The method is simple, easily applied to robotic, automated manufacturing methods, and is otherwise superior to those manufacturing methods heretofore used or proposed.
From the foregoing, it will be apparent to the reader that one important and primary object of the present invention resides in the use of a novel method for treating a workpiece to reduce fatigue stress degradation of the part while in service, and to novel tool shapes for achieving such results.
Another objective of my method, and my novel tools useful for carrying out the method, is to simplify the manufacturing procedures, which importantly, simplifies and improves quality control in the manufacture of parts having an improved fatigue life.
Other important but more specific objects of the invention reside in the provision of an improved manufacturing process for enhanced service life metal parts subject to fatigue stress, as described herein, which:
eliminates the requirement for creating a starting hole, as well as tooling and labor costs associated therewith;
eliminates the requirement for purchase, storage, and maintenance of mandrels;
eliminates the requirement for purchase, storage, and maintenance of split sleeves;
eliminates the need for disposal of split sleeves;
eliminates the need for lubrication and subsequent clean-up during manufacture of structures containing apertures therethrough;
enables the manufacture of a wide range of aperture diameters, in which appropriate fastener diameters can be employed;
allows the magnitude and depth of the residual stresses to be carefully controlled, by way of the amount of energy input into the stress wave;
enables process control to be established using statistical feedback into the manufacturing system, thus enhancing quality assurance;
eliminates shear tears in a workpiece that are commonly encountered in mandrel manufacturing methods;
significantly reduces or effectively eliminates surface marring and upset associated with mandrel methods, thus significantly increasing fatigue life;
is readily adaptable to automated manufacturing equipment, since manufacturing cycle times are roughly equivalent to, or less than, cycle times for automated riveting operations;
eliminates bulky hydraulic manufacturing equipment typically used in mandrel methods, and substitutes simple, preferably electromagnetic equipment;
enables aperture creation after fatigue treatment, by a single reaming operation, rather than with two reaming operations as has been commonly practiced heretofore;
is sufficiently low in cost that it can be cost effectively applied to a number of critical structures, including fuselage structures.
Other important objects, features, and additional advantages of my invention will become apparent to the reader from the foregoing and from the appended claims and the ensuing detailed description, as the discussion below proceeds in conjunction with examination of the accompanying drawing.
I have now invented, and disclose herein, an improved metal cold working process that uses stress waves to impart beneficial residual stresses to holes and to other features in parts subject to strength degradation through stress fatigue. This improved stress wave process does not have the above-discussed drawbacks common to heretofore-utilized cold working methods of which I am aware. The process overcomes the heretofore-encountered shortcomings of cold working processes. Also, it eliminates undesirable equipment necessary for the more commonly utilized alternative processes, such as the need for starting holes, for bulky hydraulic equipment, for precision mandrels, for disposable split sleeves, and for messy lubricants. Thus, it is believed that my novel method will substantially reduce manufacturing costs. In addition, my stress wave process is readily adaptable to use in automated manufacturing equipment. As a result, the unique process described herein is a major improvement over other processes in common use today, including mandrel processes.
My improved stress wave method imparts beneficial stresses using a dynamic indenter that impinges the surface of the metal, preferably in a normal direction to the surface. The action of the indenter causes waves of elastic and plastic stress to develop and propagate through the metal. In some cases a stationary indenter or an anvil is used to support thin workpiece materials. Such xe2x80x9cbacking indenters or anvils also assist in the reflection and or creation of plastic waves off or from the other side of the workpiece.
After a properly applied and focused plastic stress wave has imparted a large zone of residual stress, the area is now ready for the hole. A drill, reamer or other cutting device is positioned concentric to the impact zone from the indenter and/or anvil. When the hole is machined a small rebound of the stresses surrounding the hole occurs. Such rebound manifests itself as shrinking of the machined hole. For this reason, the cutting tools used in my stress wave method may require the use of a feature that takes into account the inward metal movement in a hole. Otherwise, the workpiece material would have the possibility of binding on the cutting tool. This could lead to short tool life or poor hole finish. For a drill or reamer, a simple solution to this requirement is to provide a back-taper feature. As a result, substantially uniform beneficial residual compressive stresses remain in finished structures.